Cheapest priced copy (The Lost Caverns of Ixalan): $15.99 at Card Kingdom, near mint, their price list as of .
The Skullspore Nexus 
Legendary Artifact · Mythic · The Lost Caverns of Ixalan · #340
This spell costs {X} less to cast, where X is the greatest power among creatures you control.
Whenever one or more nontoken creatures you control die, create a green Fungus Dinosaur creature token with base power and toughness each equal to the total power of those creatures.
{2}, {T}: Double target creature's power until end of turn.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
4 printings · $15.99 to $37.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Lost Caverns of Ixalan | 212 | Mythic | $15.99 | $16.99 |
| The Lost Caverns of Ixalan | 340 | Mythic | $16.99 | $19.99 |
| The Lost Caverns of Ixalan Promos | 212p | Mythic | $17.99 | $37.99 |
| The Lost Caverns of Ixalan Promos | 212s | Mythic | — | $16.99 |
4 rulings from Wizards of the Coast
- 10 November 2023The first step of casting a spell is to move it to the stack. If this causes the greatest power among creatures you control to change (for example, if you control a creature with "This creature's power is equal to the number of cards in your hand"), that new power will be used to determine the cost reduction.
- 10 November 2023Use the power of the nontoken creatures that died as they last existed on the battlefield to determine the base power and toughness of the token created by the middle ability.
- 10 November 2023To double a creature's power, that creature gets +X/+0, where X is that creature's power as The Skullspore Nexus's activated ability resolves.
- 10 November 2023Once you determine the cost to cast The Skullspore Nexus, you may activate abilities to pay that cost. If the greatest power among creatures you control changes while activating mana abilities, the cost to cast The Skullspore Nexus remains what you previously determined.